In the related art, PMOS (P-channel metal oxide semiconductor) transistors are widely used as switching elements of step-down switching regulators, and voltage control elements of linear regulators. In addition, power supply circuits using PMOS transistors as control elements are employed in secondary cell charging circuits, in parallel running power supply circuits in which output terminals of plural power supplies are connected to each other to supply electric power to loads, and in backup power supply circuits for executing temporary circuit backup when the main power supply is turned off.
In these power supply circuits using PMOS transistors, when a voltage from another power supply circuit is applied to an output terminal of such a power supply circuit, if the power supply connected to an input terminal is disconnected, or the power supply experiences trouble, an electric current flows in a reverse direction to the input terminal from the output terminal, namely, reverse direction flow occurs. Further, in the parallel running power supply circuit, if the output voltages of the power supplies are different, reverse direction flow also occurs.
FIG. 4 is a circuit diagram illustrating an equivalent circuit of a transistor.
The reason for the reverse direction flow is explained below with reference to FIG. 4.
As illustrated in FIG. 4, a diode D1 is connected between a source S and a substrate gate (also referred to as “back gate”) SG, and a diode D2 is connected between a drain D and the substrate gate SG. The diode D1 and the diode D2 are parasitic diodes produced when integrating the PMOS transistor into a semiconductor device.
Generally, when using a PMOS transistor as a switching element or a voltage control element of a power supply circuit, an input voltage is applied on the source S of the PMOS transistor, and an output voltage is extracted from the drain D of the PMOS transistor.
FIG. 5 is a circuit diagram showing an example of connection of the substrate gate to the PMOS transistor.
As illustrated in FIG. 5, in the PMOS transistor, the substrate gate SG is connected to the source S to short the parasitic diode D1. Hence, when the PMOS transistor is turned off, the parasitic diode D2 turns out to be inserted between the source and the drain in a reverse direction, so as to prevent conduction of a current from the source side connected to the input terminal to the drain side connected to the output terminal.
However, as described above, when a voltage is applied to the output terminal, if the power supply connected to the input terminal is disconnected, or the input voltage is lowered, the parasitic diode D2 ends up being turned on, and as a result, an electric current flows from the drain side connected to the output terminal to the source side connected to the input terminal, namely, reverse direction flow occurs. The reverse direction flow not only degrades efficiency of the power supply, but also causes a crash or a malfunction of the power supply circuit; hence, it is necessary to prevent the reverse direction flow.
In order to prevent the reverse direction flow in a parallel running power supply circuit in which plural power supply circuits are connected in parallel, usually, an OR diode is inserted between an output terminal of each of the power supply circuit and the load. For example, reference can be made to Japanese Laid Open Patent Application No. 6-105464.
In addition, also in the case of a secondary cell charging circuit, usually, a diode is used to prevent the reverse direction flow. When an NMOS (N-channel metal oxide semiconductor) transistor is used as a switching element of a step-down DC-DC converter, the NMOS transistor is able to prevent reverse direction flow. For example, reference can be made to Japanese Laid Open Patent Application No. 2002-84742.
However, since the forward voltage of a diode is about 0.1 Vf an input voltage of the diode in the forward direction has to be set to additionally include the forward voltage. This may cause high power loss in a circuit for supplying a large current, and degrade the power efficiency. When an NMOS transistor is used as the switching element, a voltage much greater than the output voltage has to be applied to the gate of the switching element in order to drive the switching element. Due to this, compared to a switching element using the PMOS transistor, one has to increase the difference between the input voltage and the output voltage, or supply the gate voltage from a voltage source having a high output voltage. However, when increasing the difference between the input voltage and the output voltage, the power efficiency degrades, and when using the high output voltage source, the circuit turns out to be complicated and becomes expensive.